1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT) apparatus that irradiates a subject with X-rays, collects projection data, and reconstructs an image from the collected projection data. In particular, the present invention relates to electrocardiographic synchronized imaging in which imaging is performed synchronously with a cardiac cycle of a heart.
2. Description of the Related Art
Conventionally, an X-ray CT apparatus irradiates a subject with X-rays and collects projection data by detecting the X-rays passing through the subject. The X-ray CT apparatus then reconstructs an image from the collected projection data. In an examination performed using the X-ray CT apparatus, a contrast medium is often used to pick up a clear image of organs and blood vessels to be imaged. However, blood flow causes the contrast medium to flow out from the organs and blood vessels with time. Therefore, when the contrast medium is used, a timing at which to irradiate an imaging subject with the X-rays becomes important.
A technology is disclosed (refer to, for example, JP-A 2004-113779 (KOKAI)) in which a CT value of a region of interest set in consecutively obtained images is successively measured, and imaging conditions (such as a tube current, a timing at which to start imaging, and a timing at which to complete imaging) are controlled based on changes with time in the measured CT value.
In recent years, heart examinations have generally been performed using the X-ray CT apparatus. When the X-ray CT apparatus is used to image a heart, the contrast medium is used and electrocardiographic synchronized imaging is performed. In electrocardiographic synchronized imaging, imaging is performed synchronously with a cardiac cycle of the heart. An electrocardiograph detects the cardiac cycle of the heart. Projection data of a specific phase range is collected at each detected cardiac cycle. To suppress radiation exposure of a subject, the X-rays are intermittently irradiated at each cardiac cycle.
Various types of heart examinations using the X-ray CT apparatus are performed depending on a purpose of examination and a region to be examined. For example, a myocardial perfusion is performed to examine myocardial blood flow. A CT angiography (CTA) is performed to check for coronary artery stenosis and the like. A cardiac function analysis (CFA) is performed to examine a function of an entire heart (heart function).
As stated above, a plurality of types of heart examinations using the X-ray CT apparatus are performed, each type having a different purpose of examination and involving a different region to be examined. Therefore, an X-ray exposure timing, a phase range of projection data required when reconstructing an image, and a temporal resolution of an image obtained through an examination differ with each examination type.
Here, the temporal resolution refers to a temporal element included in the image. As a result of improvement (shortening) in the temporal resolution when reconstructing the image from the projection data, an image that is little affected by movement can be obtained. Reconstruction methods that improve the temporal resolution are, for example, half reconstruction and segment reconstruction.
In a half reconstruction operation, the image is reconstructed using projection data collected while an X-ray tube is rotating within a range of 180 degrees plus α (α being a fan angle). Compared to when the image is reconstructed using 360-degree range projection data (full reconstruction), the half reconstruction can shorten the temporal resolution by approximately one-half.
On the other hand, in a segment reconstruction operation, pieces of projection data of a same cross-section and a same phase are extracted from pieces of projection data of a predetermined number of heartbeats. The extracted pieces of projection data are combined to form a piece of projection data of a range of 180 degrees plus α. Subsequently, the half reconstruction operation is performed. Compared to when the image is reconstructed using the 360 degree range projection data, the segment reconstruction can shorten the temporal resolution to about (180+α)/n when an n number of heartbeats are used. The segment reconstruction can further improve temporal resolution, compared to the half reconstruction. A set of pieces of projection data used in a single segment reconstruction operation is referred to, hereinafter, as a projection data set.
By considering an optimal X-ray exposure timing, a projection data phase range, and a temporal resolution for each examination, for example, in the myocardial perfusion, the myocardial blood flow may be confirmed as a purpose of examination to be checked. The region to be examined is a cardiac muscle. Therefore, the X-ray exposure timing is not particularly limited. The projection data is merely required to be of a specific phase range. Compared to blood vessels and the like, the cardiac muscle is an organ having a large structure. Therefore, the temporal resolution is not required to be high.
In the CTA, the purpose of examination is to check for coronary artery stenosis and the like. The region to be examined is blood vessels. Therefore, the X-ray exposure timing is preferably when a concentration of the contrast medium is as high as possible. The projection data is merely required to be of a specific phase range. Because the blood vessels are extremely narrow organs, a high temporal resolution is required.
In CFA, the purpose of examination is to examine heart function. The region to be examined is the entire heart. Therefore, the X-ray exposure timing is preferably when the concentration of the contrast medium is as high as possible. The projection data of all phase ranges in a cardiac cycle is required. Because the entire heart is a large organ, the temporal resolution is not required to be high.
Here, the concentration of the contrast represents the extent of contrast enhancement.
In this way, the X-ray exposure timing required when taking the image obtained through the examination, and the projection data phase range and the temporal resolution required when reconstructing the image differ from each examination type. Therefore, imaging is ordinarily performed at each examination in accordance with a type of the examination.
However, in actuality, a plurality of the examinations described above are often collectively performed during a series of electrocardiographic synchronized imaging operations in consideration of a burden of a patient that is the subject. In this case, during the series of electrocardiographic synchronized imaging operations, based on the examination type, an operator is required to measure an optimal timing while checking the concentration of the contrast medium and, at the same time, irradiate the heart with X-rays in the required phase range. The operator is also required to operate the X-ray CT apparatus to reconstruct the image from the projection data using the required reconstruction mode. Although the operation is conventionally performed manually, the operation is very complicated.
Therefore, when the examinations are collectively performed in during the series of electrocardiographic synchronized imaging operations, how to efficiently collect the projection data required in each examination and reconstruct the required image while suppressing the radiation exposure of the subject is an important issue.